The disclosure relates gas turbine engines. More particularly, the disclosure relates to thermal barrier coatings for gas turbine engines.
Gas turbine engine gaspath components are exposed to extreme heat and thermal gradients during various phases of engine operation. Thermal-mechanical stresses and resulting fatigue contribute to component failure. Significant efforts are made to cool such components and provide thermal barrier coatings to improve durability.
Exemplary thermal barrier coating systems include two-layer thermal barrier coating systems. An exemplary system includes NiCoCrAlY bondcoat (e.g., air plasma sprayed (APS), low pressure plasma sprayed (LPPS), or cathodic arc deposited) and yttria-stabilized zirconia (YSZ) (or gadolinia-stabilized zirconia (GSZ)) thermal barrier coating (TBC) (e.g., air plasma sprayed (APS) or electron beam physical vapor deposited (EBPVD)). Prior to and while the barrier coat layer is being deposited, a thermally grown oxide (TGO) layer (e.g., alumina) forms atop the bondcoat layer. As time-at-temperature and the number of cycles increase, this TGO interface layer grows in thickness. An exemplary YSZ is 7 weight percent yttria-stabilized zirconia (7YSZ).
Exemplary TBCs are applied to thicknesses of 1-40 mils (0.025-1.0 mm) and can contribute to a temperature reduction of up to 300° F. (167° C.) at the base metal. This temperature reduction translates into improved part durability, higher turbine operating temperatures, and improved turbine efficiency.
Separately, the material known as didymium is used as safety glasses in the glassblowing and blacksmithing industries due to advantageous selective light-filtering properties. Didymium oxide is comprised of refined of praseodymium and neodymium oxides that have been extracted from rare earth containing minerals (e.g., separately and then mixed). Didymium is commonly extracted from Monazite and Bastnasite. Didymium ore may be formed by extracting cerium from monazite. Praseodymium and neodymium are, for example, extracted from the ore and oxidize in the process. These oxides are combined to form didymium oxide.
Discovery of the rare earth elements took approximately 160 years beginning in 1787 with Gadolinite by C. A. Arrhenius. In 1839 C. G. Mosander discovered the element ‘Didymium’ upon extracting it from Cerite. The element's name was derived from the Greek ‘didymos’ meaning twins. In Mosander's experiments the behavior of didymium was peculiar, tracking lanthanum in some experiments wile tracking cerium in others. Scientists in the latter half of the 19th century widely suspected that didymium was actually two elements, but did not have a method of separating the compound. C. A. Von Welsbach successfully separated Didymium into its individual components in 1886. The elements were named by A. Bettendorf: Praseodymium the “green twin” and Neodymium the “new twin”.
In modern mining techniques, separation of the rare earth bearing heavy mineral deposits is accomplished through a series of processing sequences that exploit the small differences in mass, magnetic susceptibility, and electrochemical properties. The number and order of operations is predicated on the source of the deposit and purity of the heavy mineral deposits.
Recovery of REO from either mineral involves a complex series of operations by which the rare earths are separated from the radioactive components of the mineral. Monazite is dissolved into solution using caustic soda (NaOH). This mixture is then washed and filtered. Two byproducts are evolved from this step are a mixed rare earth (RE)-thorium-uranium hydroxide and a filtrate containing sodium phosphate. Hydrochloric acid is then added to the RE-Th-U hydroxide solution. The solution is subsequently filtered and washed to separate the radioactive constituents, uranium and thorium, from the desired RE components. The filtrate from this process is neutralized by chemical processing to yield a RE chloride mixture. The remaining liquid fraction is treated either with caustic soda and/or sodium bicarbonate to form additional RE hydroxide or RE carbonate. Similar processing is also used to digest bastnasite RE minerals; however different processing steps are utilized desired RE elements.
Cerium is the first RE to be removed from the mixture of lanthanide elements. This can be accomplished by drying the rare earth hydroxide mixture and then oxidizing cerium (III) to cerium (IV) in the presence of ozone. The mixture is then dissolved in nitric acid. The subsequent mixture is filtered leaving a cerium-free RE solution and cerium (IV) dioxide filtrate. The remaining mixture of rare earths can be further processed by a series complex ion exchange and digestion methods to separate the mixture into each elemental constituent. Praseodymium and Neodymium oxides are then blended together in exact ratios to form the mixtures as described in Table III below. See, e.g., Extractive Metallurgy of Rare Earths, C. K. Gupta and N. Krishnamurthy, CRC Press 2004.
U.S. Pat. No. 6,863,999 identifies use of a lanthanum monazite phosphate in a thermal barrier coating.